1. Field of the Invention
This invention relates to production method for a laminate type dielectric device such as a laminate type capacitor, a laminate type piezoelectric actuator, etc, and an electrode paste material for forming electrode layers in the dielectric device.
2. Description of the Related Art
Laminate type dielectric devices produced by alternately laminating dielectric ceramic layers having various dielectric properties and electrode layers have been widely used in the past. Known electrode materials for forming these electrode layers include Pt, Pd, Ag, Ni, Cu, their mixtures and their alloys.
The problems encountered in producing the electrodes vary from electrode material to electrode material. Silver (Ag), for example, has a high electric conductivity and is relatively economical. However, Ag has a low melting point of 980° C., is likely to invite migration and therefore has low reliability.
In contrast, palladium (Pd) is expensive but has a high melting point. Pd has therefore been used in the form of an Ag—Pd metal material to suppress migration and to improve the melting point of the electrode material (refer to Japanese Unexamined Patent Publication (Kokai) No. 5-304043).
The addition of Pd can suppress migration, it is true, but bonding is not sufficient between the electrode material and a ceramic material. Various measures have been taken to cope with this problem as described in Japanese Unexamined Patent Publications (Kokai) Nos. 5-304043 and 8-255509.
Nickel (Ni) involves problems such as the occurrence of cracking and deformation and so-called “islanding” of the electrode due to superheating (rapid sintering of Ni at a temperature exceeding 1,000° C.). To cope with the problems, Japanese Unexamined Patent Publication (Kokai) No. 5-55077 proposes to mix Ni and NiO, and Japanese Unexamined Patent Publication (Kokai) No. 6-290985 proposes to add an oxide of a rare earth element.
Among them, the prior art technology using Ag aims at solving the problem that results from Ag, and the prior art technology using Ni, at solving the problem of superheating of Ni that is peculiar to Ni. However, these technologies for producing the electrode are expensive due to the expensive additives and the machining cost. So long as these materials are used, reduction of the production cost of the laminate type dielectric device that has widely been used will be difficult. Further, technologies in the case of Ag and Ni cannot be easily applied to Cu.
On the other hand, copper (Cu) could be a promising material among base metals as the economical electrode material. In connection with Cu-containing paste materials or electrodes, a technology is known that suppresses the occurrence of cracking resulting from oxidation expansion of Cu by mixing Cu and Cu2O at a suitable proportion (Japanese Unexamined Patent Publication (Kokai) No. 5-283274). Further, there are also known a method that forms a complex by using an organic phosphorus compound and a metal (Cu) ion, and baking the complex to assist sintering of ceramics and to make the film thickness uniform (Japanese Unexamined Patent Publication (Kokai) No. 5-242724), and a method that suppresses the occurrence of warp and cracking of the device by limiting the Cu content to 40 to 70 wt % to reduce the coating thickness (Japanese Unexamined Patent Publication (Kokai) No. 5-234414).
Still another known technology uses a paste material containing 40 to 60 wt % of Cu powder having a mean particle diameter of 0.5 to 2 μm and a particle size distribution of 0.3 to 4 μm and bakes the paste material to a film thickness of 1 to 3 μm so as (1) to suppress the occurrence of voids between the device and the electrode and inside the electrode, (2) to suppress the occurrence of breakage of the electrode resulting from warp of inner and outer electrodes, (3) to prevent deformation of a ceramic component itself and (4) to prevent inferior contact between the inner electrode and the outer electrode (Japanese Unexamined Patent Publication (Kokai) No. 5-190375). In this way, the methods and objects of using Cu electrodes are very diversified.
As an example relating to a Cu electrode or a Cu type paste material, Japanese Unexamined Patent Publication (Kokai) No. 5-275263 describes the addition of at least one kind of the principal components of the ceramic layer or a material having substantially the same composition as the ceramic layer (hereinafter called the “base” of the ceramic layer or the “cooperative material”). The cooperative material designated in this prior art reference needs a specific processing of adding inorganic powder the surface of which is coated with a metal of the same kind as metal powder of the electrode. The object of this technology is to inhibit sintering of the metal so as to prevent discontinuity of the electrode and the increase of the resistance. To solve the fundamental problem that sintering of Cu proceeds more quickly than sintering of the ceramic material, this reference attempts to retard sintering by impeding sintering of the metal and to avoid discontinuity of the electrode.
Another example of the addition of the cooperative material of the ceramic layer is described in Japanese Unexamined Patent Publication (Kokai) No. 64-89311. This technology relates to an internal electrode paste for a laminate ceramic body, and describes the addition of the same compound as the ceramic material of the ceramic body. However, the construction of the electrode paste material is not CuO, as used in the present invention as will be described later, but is Cu2O.
In the case of the paste material consisting of CuO, large volume shrinkage occurs when CuO is reduced to metallic copper with the result of the occurrence of cracks in the sintered body, and de-lamination due to the occurrence of voids between the ceramic layer and the electrode layer. To solve this problem, the reference describes Cu2O as a substitution. The cooperative material of the ceramic layer is utilized to improve bondability between the inner electrode layer and the ceramic layer. In other words, this reference solves the problem of shrinkage of the Cu oxide in the electrode paste material by using Cu2O in place of CuO, and utilizes the cooperative material of the ceramic layer to improve bondability between the ceramic layer and the electrode layer.
Another example that uses the cooperative material of the ceramic layer is described in Japanese Unexamined Patent Publication (Kokai) No. 2-22806 that relates to a laminate ceramic capacitor.
In the case of this laminate ceramic capacitor, too, the reference describes that the cooperative material of the ceramic layer is to impart de-lamination resistance. Examples of this reference use a copper paste (metal paste) but do not use CuO. The reference does not present the technical significance of the addition of the cooperative material of the ceramic layer.
Production of a laminate type dielectric device is not easy when the copper (metal) paste described in the Laid-Open Patent Publication described above is used.
For, the production process for producing the laminate by applying the paste material includes (1) degreasing, (2) metallizing, that is, a reducing step of the electrode material from CuO to Cu when the laminate is produced by using a CuO paste material, and (3) baking.
When the copper (metal) paste as the electrode paste material is used to produce the laminate, degreasing must be carried out in the reducing atmosphere to avoid peeling resulting from oxidation of copper (involving expansion). Degreasing is the step of dissolving and removing a binder, and the like, and when chemically expressed, it is the oxidation of carbon, and the like. Therefore, when degreasing is carried out in the reducing atmosphere, a longer processing time is necessary than degreasing in normal open air because the oxygen content is small.
Unless degreasing is sufficiently done, the remaining carbon reacts with oxygen and exerts adverse influences in the subsequent baking step in the reducing atmosphere. For example, because carbon is likely to remain at the center of the laminate, a difference in the atmosphere occurs between the outer peripheral portion and the center portion during reduction/baking even inside the same laminate type dielectric device, and the drop of displacement performance, or the like, partially develops with diffusion resulting from oxidation of the electrode or the reduction of the ceramic layer. On the other hand, complete degreasing is not practical because a longer time is necessary.
For the reasons described above, a paste material of copper oxide capable of being degreased even in the oxidizing atmosphere is advantageous as the electrode paste material for forming the electrode layers of the dielectric device produced by alternately laminating ceramic layers containing a lead element as the constituting component and the electrode layers.
As a matter of fact, when the CuO paste material is applied and laminated with the ceramic material, is then degreased, metallized (reduction of CuO of the electrode material to Cu) and baked, peeling does not occur between the electrode portions and the ceramic layers but voids develop in the electrode portions. When the CuO content in the CuO paste material is increased to solve this problem, the voids in the electrode portions can be eliminated and the continuous electrode layer can be formed, but voids develop in the ceramic layer.